The flashlight, a small portable device consisting of a small incandescent bulb, a battery and a normally open switch contained in a small plastic or metal casing, has been known to society for many years. Improvements have been made in the construction and operation, for example, the carbon-zinc cell has been almost completely replaced by the alkaline cell while the incandescent bulb has been improved by using xenon as a filling gas and by other similar changes.
However, there remain two fundamental shortcomings of the present day flashlight. One is due to the characteristics of the tungsten filament lamp or bulb which physically approximates to a black body radiator. Such a black body radiates light proportional to the fourth power of the filament temperature. This means that when the batteries are fresh, there is an adequate flux of light, but as batteries age, the light becomes rapidly weaker and more reddish due to the lower operating temperature of the filament. Although alkaline batteries have much flatter voltage versus time discharge characteristics than the earlier carbon-zinc version, increasing internal resistance during discharge still results in steadily decreasing terminal potential and hence light output over the operating life.
A second shortcoming is the inability to reduce the light output at will to either conserve battery life or to avoid an excessive light which decreases dark adaptation. This latter problem afflicts pilots flying at night who need to use a flashlight for finding maps and other objects in the cockpit, nursing staff or security guards conducting night inspections, and other similar users. As of today, this is alleviated by fitting red filters over the flashlight lens, but this reduces visual intensity without any compensating increases of battery life. Also there is not control of the intensity of the red filtered light, if adequate with a new battery it rapidly becomes insufficient as the battery ages.